This invention relates to raster scanned imaging systems and, more particularly, to methods and means for electronically converting from one resolution to another. Accordingly, one of the more important applications of the present invention is in matching raster input and output scanners which operate with different resolutions.
By definition, an original document (whether it be a written or printed page, a drawing, a photograph, or anything else of like character) is a two dimensional, generally continuous pattern of reflectance. As is known, "raster input scanning" is a one dimensional sampling process for serially converting or re-mapping the image or information content of a document into a one dimensional video signal suitable for electronic processing and/or transmission. Conversely, "raster output scanning" is the complementary process for serially reconstructing or otherwise printing a document in response to such a video signal. Indeed, raster input and output scanning are in widespread commercial use in facsimile systems, to name just one of the better known types of "raster scanned imaging systems."
Resolution is one of the basic parameters of any raster input or output scanner. Typically, it is expressed in terms of a number of scan lines/inch vertically (i.e., the scan pitch) by a number of picture elements/inch horizontally (i.e., the sampling density). For example, the Xerox Telecopier 200 facsimile transceiver, which is manufactured and sold by Xerox Corporation, offers resolutions of 96 .times. 96 (at document transmission times of three and six minutes), 64 .times. 96 (at a document transmission time of four minutes), and 77 .times. 80 (at a document transmission time of two minutes).
Compatible raster input and output scanners are generally selected to have identical resolutions. A case in point is provided by the Xerox Telecopier 200 transceiver where the 96 .times. 96 six minute resolution and the 64 .times. 96 four minute resolution were included to provide backward compatibility with existing transceivers, including the Xerox Telecopier 400 units, which are also manufactured and sold by Xerox Corporation. To that end, provision was made in the Telecopier 200 transceivers for electromechanically varying the scan pitch (i.e., the vertical resolution) as a function of the document transmission time selected.
An attractive alternative is to carry out the resolution conversion electronically. Heretofore, however, that approach has been largely disregarded because the available techniques tend to introduce too much conversion error.